Mercaptan extraction



Feb. l2, 1952 T. B; TOM rl-:T AL

MERCAFTAN EXTRACTION Filed Aug. 7, 1948 Patented Feb. 12, 1952 MERCAPTAN EXTRACTION Theodore B. Tom, Hammond, Ind., and John A.` Bolt, Chicago, Ill., assignors to Standard Oil Company, Chicagmlll., a corporation of Indiana Application August 7, 1948, Serial No. 43,093

4 Claims.

This invention relates to a process of separating mercaptans from relatively high boiling oilsV such as fuel oil distillates of intermediate boiling range. The invention relates particularly to a process of improving the burning characteristics of heater oil derived from petroleum by distillation of crude oils and cracked petroleum products. The invention is illustrated by a drawing which shows diagrammatically an apparatus suitable for carrying out the process.

In the refining of heater oils a serious problem has been encountered with formation of carbon or coke in oil burners, especially in domestic sleeve-type space heaters in which the vaporizing grooves of the burner fill with carbon so that oil flow is retarded and shorter periods of operation between shut-downs for cleaning result. It has been found that one important cause of deposits is a small amount of high-boiling sulfur compounds dissolved in the oil and allowed to remain after the refining process.

When heater oils containing mercaptans are refined by the usual processes in which the oils are sweetened to remove the mercaptans because of their objectionable odor, a part or all of the mercaptans are converted to disulfides which have a considerably higher boiling range than that of the oil from which they are derived. Studies on burning performance have shown that these high-boiling disuliides are particularly undesirable in burning oils destined for use in critical space heaters as shown below.

Fifty gallon quantities of several heater oils of varying disulde content were burned in a Jungers Model C sleeve-type space heater. In this test conducted at a draft equivalent to 0.03 inch water pressure and at 0.3 gallon per hour ow rate, an oil producing 6 grams or less of carbondeposits on the heating surfaces in the course of a 50 gallon run is considered acceptable. The influence of high boiling disuliides can be seen in the table below. Y,

Heater oil burning quality The mercaptan number, sometimes called the copper number, is the milligrams of mercaptan sulfur per ml. of oil, generally determined by titration with a standardized copper salt solution.

5 The disulfide number is the milligrams of disulfide sulfur per 100 ml. of oil. It is determined by reducing the disulfides (Zn-l-HCI) to mercaptans f and then determining the mercaptan number.

As demonstrated by the above data. it has become necessary either to remove the disuldes from the oil or prevent their formation in rening operations and; even in storage where disuliides are produced by atmospheric oxidation of mercaptans. This invention is concerned with the first method of solving the problem of producing heater oils with satisfactory burning quality by extraction of the mercaptans from the oil before they have become oxidized to disuliides.

Numerous processes have been devised heretofore to extract mercaptansfrom petroleum oils, and particularly from light petroleum distillates such as gasoline and napthas. The mercaptans found in these light distillates are quite reactive with caustic alkalies, and the lower molecular weight mercaptans, such as methyl and ethyl mercaptans, can be removed by washing with aqueous solutions of caustic alkalies, particularly sodium and potassium hydroxide. The removal of the mercaptans found in gasoline has been effected by extracting with caustic solutions in various solvents, particularly methanol, and the methanol-caustic process for removing mercaptans from gasoline has been developed on a commercial scale. However, when it was attempted to remove the heavy mercaptans from heater oils by this process, it was found that because of the higher oil solubility and/or the lower acidity of these heavy mercaptans, only partial separation of the mercaptans could be effected.

According to our invention, we have Vnow found that the heavy mercaptans occurring in heater oil can be extracted by a caustic solution in The amount of acid which we g 35 per cent, based on the total weight of the lextraction solution. The proportion by weight of aqueous caustic in the total solution Yshould be about 20% to about 80%, the weightper `cent of caustic in the aqueous caustic portion of the solution being within the range of from about 35% to 60%. The remainder of the extraction solution should be substantially methanol. Relatively less amounts of water arev employed when KOH is used than in the case where NaOH is used.

In general, KOH solutions are found to be somewhat more effective in removing mercaptans than are -NaOH solutions. It is important to avoid contacting the caustic solutions with carbon dioxide which neutralizes part of the caustic and uno balances the composition of the solution. Separation of carbonates also creates a problem. The

operationof the process can be seen by referring :.1

to the drawing.

According Ato the drawing, heater oil is charged to the Aprocess byline I leading to mixer II and settler I2 in which an alkali solution is employed for the primary purpose of removing HzS and CO2. For this purpose a dilute solution of caustic 4 volves the treatment of huge quantities, it is essential to recover the reagents employed in the process, specifically caustic alkali methanol and solutizer. The recovery operation imposes numerous problems which we have solved in the manner described hereinafter. The rich causticmethanol-organic salt solution, sometimes called the spent caustic solution, is withdrawn from the rst extraction stage by line 32 leading to heat exchanger 33 and thence to heater 34 where it is heated to a temperature of about 210 F. The caustic methanol solution then passes by line 31 t0 caustic `stripper 38 in which methanol and mercaptans are substantially completely removed from the caustic solution and solutizer. Heat is supplied for. reboiling the stripper by heater 39 sodais satisfactory. It can be recycled by pump `v I3 from settler I2 to mixer I I and discarded when spent.

The HzS-free heater oil is conducted by line I4 tomixer .I5 and settler I5., the rst mercaptan l that a satisfactory extraction can be obtained e to condenser 49 and thence to receiver 50, a porwith only two to ve stages and where KOH solutions are used only two to four stages are required. The drawing shows an operation with two extraction stages.

In mixer. I5,v caustic-methanol-organic salt solutions are vintroduced vby lines I1 and I8, are I mixed withV the oil I5 and settled .in I6. The caustic .solution from settler I6 canbe recycled by pump I9 to mixer I5. Separated oil from the rst extraction stage is conducted byline 2U to mixer 2I and settler 22, caustic solution 'being supplied to mixer 2l by line 23 and recycle line 24, separated Vcaustic from settlei` 22 being recycled by pump 25. Separated oil from settler 22, substantially free of mercaptans is conducted by line 26 to mixer 21 and settler 28 in which the oil is washed with water to recover a small amount of methanol dissolved therein.. Because o'f the heavy character of the heater oil, the amount of methanol soluble therein is quite small, usually less than about 2% when employing caustic-methanol organic salt solutions described hereinabove. Wash water is supplied to mixer 21 by line 29. Wash water from settler 28 can bev recycled by pump 30 to mixer 21 to increase the volume of water for contacting the oil stream. Finished, substantially mercaptan-free heater oil product is withdrawn from settler 28 by line 3l.

Inasmuch as heater oil is 'a relatively lowpriced product and the refining operation intionator 41 and steam or water are introduced at a low point in the stripper for aiding in removing the cercaptans from the caustic solution, superheated steam .being preferred. The temperature of the caustic solution in the base of the stripper 38 is preferably of the order 'of 250 to 350 F. Water supplied by line 40 to heater 39 is converted to steam therein and serves to aid .the removal of heavy mercaptans from the caustic solution, sufcient heat being supplied by heater 39 to superheat the steam, preferably to a, temperature of about 400 to'600" F.

From the top of stripper 38 vapors of methanol, water and mercaptans are withdrawnby line 4I leading to condenser 42 in which the water, methanol and mercaptans are largely con- .densed. The organic acids, being rmly combined with the caustic, remain behind in the still residue with the strong caustic. Some of the cercaptans in excessA of their solubility in the methanol-water solution are withdrawn by line 43 from drum 44 While the methanol-water solution is pumped by pump 45 thru heater 46 into frac- Heat for reboiling the fractionator is supplied indirectly by heater-48. Alternatively, the vapors from 38 may be conducted by line ha directly to fractionator 41 without condensation. From fractionator 41, vapors of methanol, substantially free of water, are conducted overhead tion of the methanol being recycled by line 5I as reflux to column 41. Some mercaptans tend to distill over with the methanol but with a highboiling stock such as heater oil, the amount is usually less than 1%.

Methanol from receiver is conducted'by line 52 and thence by line 5 4 to the stage extraction system. From the bottom of caustic stripper 38 the .solution of hot caustic, substantially'free of mercaptans, is conducted by line 55 back to the extraction system.

Hot caustic solution containing, for example, 40 to 10 per cent KOH and organic salts, is'passed to exchanger 33 and thence thru cooler 56 to line 23 and mixer 2| as hereinbefore described. The rate of ow of caustic solution in vrecycle line y55 and methanol in line 52 is controlled to maintain the proper composition of the extraction solution. Water may also be injected by line 51 into the recycle solution in line 23 to regulate the composition and extraction'eiciency. In order to prevent separation of alkali metal .salts of organic acids in exchanger 33 and cooler56, a small amount of water and/or methanol can be added to the caustic solution by valved line 58 and 59 respectively.

Water mercaptans containing not more than about 1 to 5 per cent methanol are withdrawn from fractionator 41 by line '60 leading to settler 6| in which an oily upper layer of mercaptans is separated and withdrawn by line 62. The water, substantially mercaptan free, is conducted by lines '63 and 29 to washer 21. A portion of the water controlled by valve 64 in line 65 may be diverted to the methanol-mercaptan stream entering separator 44 where it aids in effecting a rough preliminary separation of mercaptans from the methanol before charging to the methanol fractionator 4l'. Additional reagents required for maintaining the extraction solution in a high state of effectiveness can be added to the system from time to time to compensate for losses. Thus additional water can be added to drum 6| and additional caustic thru line 51, while methanol may be added to drum 50. In our preferred operation, we employ potassium ethoxypropionate with KOH and methanol.

The following specification described a typical heater oil refined by our process:

Gravity API 39 to 43 Color 8 to 18 Saybolt Universal Flash TCC 115 to 150 Mercaptan No 5 Copper strip corrosion- 3 hours at 212 F Negative Distillation ASTM:

Initial 332 F.. 10% 360 F. 50% 438 F. 90% 527 F. Maximum 574 F.

The following examples describe the separation of mercaptans from two lots of heater oil in one, two and three extraction batches using a fresh solution for each contact. The original heater oils before extraction had mercaptan numbers of 100 and 73 corresponding to approximately 0.11% and 0.09% mercaptan sulfur respectively. The oils were extracted with the caustic solution in the ratio of iive volumes of oil to one volume of solution. The time of contact in each extraction stage was fifteen minutes. The temperature of extraction was approximately 90 F. The composition and efficiency of the extraction solutions are given in the following table:

1. The process of removing high boiling mercaptans from a petroleum distillate boiling in the range of about 350 F. to about 600 F. which process comprises intimately contacting said distillate with a treating agent consisting essentially of water, alkali metal hydroxide, methanol and an alkali metal salt of an alkoxyalkylcarboxylic acid having about 4 to 8 carbon atoms per molecule, the amounts of water and alkali metal hydroxide being such as to give an aqueous alkali metal hydroxide solution containing about 40 to weight per cent alkali metal hydroxide, the amount of methanol being in the range of about 30 to 50 volumes of methanol per 100 volumes of the aqueous alkali metal hydroxide solution and the amount of alkali metal salt of alkoxyalkylcarboxylic acid being in the'range of about 5 to 35 weight per cent based on total treating agent.

2. The process for removing high boiling mercaptans from a petroleum distillate boiling in the range of about 350 F. to about 600 F. which process comprises intimately contacting said distillate with a treating agent which consists essentially of potassium hydroxide, water, methanol, and a potassium salt of an alkoxyalkylcarboxylic acid having from about 4 to 8 carbon atoms per molecule, these components being present in substantially the following amounts by weight:

Per cent Potassium hydroxide 10-40 Water 11-34 Methanol 15-47 Potassium salt of alkoxyalkylcarboxylic acid 10-31 the amount of water and potassium hydroxide being such as to give a potassium hydroxide concentration of 40 to 55 weight per cent.

3. The process of claim 2 wherein the acid is ethoxyacetic acid.

4. The process of claim 2 wherein the acid is ethoxypropionic acid.

THEODORE B. TOM. JOHN A. BOLT.

REFERENCES CITED The following references are of record in the file of this patent:

Mercaptan Number No. Caustic Solution Compositionsl After Before Exm- 1st 2nd ard Extn. Extn. Extn.

1 11% KOH, 12% H10, 17% Meeuw-. 100 39 21--.-- 50% KoH,40% H10. 10% MeoH 100 p 31 a 11% KOH, 10% H10, 20% Kezhoxy- 100 24 11 4 acetate, 47% MeOH. 4 10% KoH,15% mo.31%Kmethoxy 100 3o 14 s nropionate. 44% MeOH. s 11% KoH.1e% 11,0, 27% K ethoxy- 100 1s a 1 propionate, 46% MeOH. s 13.5% Kon, 11.5% mo, 30% K 7s 1o 3.5

ethoxvurnpionate. 45% MeOH. 7 40.5% KOH, 34.5% H10. 10% K 73 7 2 ethoxypropionate, 15% MeOH.

1 Per cent by wt. based on total reagent solutions. f.- I Blank runs-no organic salt.

It will be noted from these results that the UNIT combination of methanol and potassium hydrox- ED STATES PATENTS ide solution with ethoxypropionate is capable of Number Name Date reducing the mercaptan content of the heater oil 2,131,036 Wilson N0v 21, 1939 to a negligible amount, 2.0 mercaptan number, 2,297,621 Henderson et al. Sept. 29, 1942 corresponding to about .022% mercaptan sulfur 2,309,651 McCullough et al. Feb. 2, 1943 after only two extractions with fresh solution for 2,317,053 Henderson Apr. 20, 1943 each Contact 2,335,347 McNamara Nov. 30, 1943 Having thus described our invention what we 2,381,859 Ayers et al. Aug. 14, 1945 claim is: 2,439,670 Oosterhout Apr. 13, 1948 

1. THE PROCESS OF REMOVING HIGH BOILING MERCAPTANS FROM A PETROLEUM DISTILLATE BOILING IN THE RANGE OF ABOUT 350* F. TO ABOUT 600* F. WHICH PROCESS COMPRISES INTIMATELY CONTACTING SAID DISTILLATE WITH A TREATING AGENT CONSISTING ESSENTIALLY OF WATER, ALKALI METAL HYDROXIDE, METHANOL AND AN ALKALI METAL SALT OF AN ALKOXYALKYLCARBOXYLIC ACID HAVING ABOUT 4 TO 8 CARBON ATOMS PER MOLECULE, THE AMOUNTS OF WATER AND ALKALI METAL HYDROXIDE BEING SUCH AS TO GIVE AN AQUEOUS ALKALI METAL HYDROXIDE SOLUTION CONTAINING ABOUT 40 TO 55 WEIGHT PER CENT ALKALI METAL HYDROXIDE, THE AMOUNT OF METHANOL BEING IN THE RANGE OF ABOUT 30 TO 50 VOLUMES OF METHANOL PER 100 VOLUMES OF THE AQUEOUS ALKALI METAL HYDROXIDE SOLUTION AND THE AMOUNT OF ALKALI METAL SALT OF ALKOXYALKYLCARBOXYLIC ACID BEING IN THE RANGE OF ABOUT 5 TO 35 WEIGHT PER CENT BASED ON TOTAL TREATING AGENT. 